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| Experiment 2 | pKa of Bromocresol Green (BCG) |
Notes
These notes contain procedure hints. They also describe some of the "behind the scenes" chemistry that occurs during the experiment.
To verify my math begin by re-writing the equation:
Next, use the various B-L laws that we had obtained to substitute for each of the absorbances:
This equation can be simplified by recalling that the sum of the equilibrium concentrations of HA and A- equals the initial concentration. In other words, Co = cHA + cA. It is also useful to notice that the sample thickness, b, can be factored from the other terms and removed.
Now the rest is simple:
The pipets you will use should provide volumes with a precision of +/1 0.01 mL provided that you fill and drain them correctly. The volumetric flask needs to be clean, but not dry. (You are going to add water to dilute the reagents so a little water in advance won't hurt anything.)
Use the pipet provided with the reagent bottle. The tip has been squeezed so that it delivers smaller drops than normal.
pH electrodes vary considerably in their age and quality. Make sure that you calibrate your electrode carefully before you begin making measurements. It is also wise to test for electrode drift by re-checking some of your early measurements.
Use a Pasteur pipet, not a wash bottle, to fill water to the mark. Pipets give you more control.
Student success with burets has been mixed. Feel free to use air pipets to deliver measured volumes of Acidic BCG and Basic BCG.
This procedure assumes a standard cell (volume = 3 mL). If you use a mini-cell (sample volume ≤ 1 mL), you do not need to return the liquid from the cell to the sample tube.
